Allow skipping CSRF check for Referer header

[Aaron Hill]

Currently, Django's CSRF middleware will reject any 'non-safe' HTTPS request that lacks a Referer header: ​https://github.com/django/django/blob/22e8ab02863819093832de9f771bf40a62a6bd4a/django/middleware/csrf.py#L242

However, some users may prevent their browsers from sending the Referer header, due to privacy concerns. These users are unable to submit 'non-safe' requests (e.g. POST requests) on HTTPS-enabled Django-powered website that use CSRF protection.

For some websites, checking the Referer header may provide no added security benefit. For example, an HSTS-preloaded website which controls all of its subdomains has nothing to gain from this check - there are no untrusted subdomains which can mount an attack, and HSTS prevents an HTTP MITM attack.

To allow these websites to provide more flexibility to their users, Django should support disabling this CSRF Referer check. This could be done through a new setting, e.g. ' CSRF_REFERER_CHECK' (defaulting to 'True' to avoid breaking existing sites).

[Tim Graham]

Please make your proposal on the DevelopersMailingList as the addition of new settings requires a consensus.

[Mariusz Felisiak]

Please first start a discussion on the DevelopersMailingList, where you'll reach a wider audience and see what other think, and ​follow the guidelines with regards to requesting features. Thanks.